I haven't seen this mentioned here, so I thought I would. PC Mag did a screen comparison with actual measurements of the screens on these phones. Short answer is the iPhone 4 was the best overall. Something that surprised me is that except for the Droid X, all the other Android phones use 16 bit color. The Droid X and iPhone both have 24 bit color display. Also, they did not compare pixel density, either, but only how well each phone did with the display measurements.
http://www.pcmag.com/article2/0,2817,2365915,00.asp
http://www.pcmag.com/article2/0,2817,2365915,00.asp